Secondary cell and method of operating the secondary cell

ABSTRACT

The invention provides a secondary battery system that allows a high overload operation regardless of discharging condition during a steady operation, and an operating method thereof. The secondary battery system comprises first tanks  31, 32  for reserving electrolytes required for a steady operation, and second tanks  33, 34  for reserving electrolytes required for an emergency operation. Valves  41 - 48  are opened and closed for allowing selective switching between the electrolytes in the first tanks  31, 32  and the electrolytes in the second tanks  33, 34  to circulate the selected electrolytes through a cell stack  100 . The electrolytes reserved in the second tanks  33, 34  are electrolytes having a proportion of a quantity of active material produced in a charging reaction to a total quantity of active material of not less than 50%.

TECHNICAL FIELD

[0001] The present invention relates to a secondary battery system and,more particularly, to a redox flow battery that can allow a highoverload operation even in case of emergency such as electric powerfailure.

BACKGROUND ART

[0002]FIG. 6 is an explanatory view showing an operating principle of aredox flow battery. As illustrated therein, the redox flow battery has acell 1 separated into a positive electrode cell 1A and a negativeelectrode cell 1B by a membrane 4 of an ion-exchange membrane. Thepositive electrode cell 1A and the negative electrode cell 1B include apositive electrode 5 and a negative electrode 6, respectively. Apositive electrode tank 2 for feeding and discharging positiveelectrolytic solution to and from the positive electrode cell 1A isconnected to the positive electrode cell 1A through conduit pipes 7, 8.Similarly, a negative electrode tank 3 for feeding and dischargingnegative electrolytic solution to and from the negative electrode cell1B is connected to the negative electrode cell 1B through conduit pipes10, 11. Aqueous solution containing ions that change in valence, such asvanadium ion, is used for the positive and negative electrolytes. Theelectrolyte containing the ions is circulated by using pumps 9, 12, tocharge and discharge the electrolyte with the change in ionic valence atthe positive and negative electrodes 5, 6. When the electrolytecontaining the vanadium ions is used, the following reactions occur inthe cell during the charge and discharge of electricity:

[0003] Positive electrode: V⁴⁺→V⁵⁺+e⁻ (Charge) V⁴⁺←V⁵⁺+e⁻ (Discharge)

[0004] Negative electrode: V³⁺+e⁻→V²⁺ (Charge) V³⁺+e⁻←V²⁺ (Discharge)

[0005]FIG. 7 is a diagrammatic illustration of construction of a cellstack used for the redox flow battery mentioned above. This type ofbattery usually uses the construction which is called a cell stack 100comprising a plurality of cells stacked in layers. Each of the cells hasthe positive electrode 5 and the negative electrode 6 which are made ofcarbon felt and disposed at both sides of the membrane 4. It also hascell frames 20 disposed at the outside of the positive electrode 5 andat the outside of the negative electrode 6, respectively.

[0006] Each of the cell frames 20 has a bipolar plate 21 made of carbonplastic and a frame 22 formed around the outside of the bipolar plate21.

[0007] The frame 22 has a plurality of holes which are called manifolds23A, 23B. The manifolds 23A, 23B are arranged to form flow channels ofthe electrolytic solutions when a number of cells are stacked in layersand communicate with the conduit pipes 7, 8, 10, 11 of FIG. 6.

[0008] The redox flow battery is usually used with the aim of allowingload-leveling through the steady operation that electricity isdischarged during daytime when more electric power consumption isrequired and electricity is charged (stored) during nighttime when lesselectric power consumption is required. For the load-leveling, highefficient operation of the battery is desirable from the viewpoints ofenergy saving and cost reduction. On the other hand, in case ofemergency such as an instantaneous electric power failure in the steadyoperation, it is desirable to bypass the efficient operation in favor ofhighest possible overload operation of the battery. It should be notedhere that the term “overload operation” means operation at an output inexcess of a rated output, and the term “rated output” means an output atwhich energy efficiency during the charge/discharge of electricityreaches a design value or more. In general, the rated output is oftenset at about 80% of the maximum output.

[0009] The redox flow battery can allow a comparative high overloadoperation when it is in the fully charged state, but it cannot allow theoverload operation substantially when electric energies stored in theelectrolyte are less at the end stage of discharge or after the end ofdischarge.

[0010] This is because when the electrolyte is high in state of charge,the redox flow battery can allow a high overload output, while however,when the electrolyte drops in state of charge, the voltage is reduced,making it hard for the redox flow battery to allow the overload output.The expression “the electrolyte is high in state of charge” indicatesthe state that when a vanadium-based electrolyte is used for theelectrolyte, the electrolyte for the positive electrode has a high ratio“(concentration of quinquevalent vanadium ions)/(concentration oftetravalent+quinquevalent vanadium ions)” and the electrolyte for thenegative electrode has a high ratio “(concentration of bivalent vanadiumions)/(concentration of bivalent+trivalent vanadium ions)”.

[0011] For allowing this overload operation, the conventional redox flowbatteries require a largely increased amount of electrolyte and alsorequire that the electrolyte be constantly kept high in state of chargeeven after discharging in the steady operation. However, theload-leveling operation requires a fluid volume of electrolytecorresponding to its capacity of a few hours or more, and to obtain theconstant increase in the state of charge by increasing the fluid volumeof electrolyte requires a significantly large amount of electrolytes.

[0012] Accordingly, it is a primary object of the present invention toprovide a secondary battery system that can allow a high overloadoperation even in the discharge state during the steady operation, andan operating method thereof.

DISCLOSURE OF THE INVENTION

[0013] In order to accomplish the object mentioned above, the presentinvention is constructed so that electrolytes having a high state ofcharge for an emergency operation are reserved, in addition toelectrolytes for a steady operation, so that when an accident such aselectric power failure occurs, the electrolytes for emergency operationare fed to a battery cell reliably.

[0014] Specifically, the present invention provides a secondary batterysystem comprising at least one set of first tanks for reservingelectrolytes required for a steady operation, at least one set of secondtanks for reserving electrolytes required for an emergency operation,and switching means for allowing selective switching between theelectrolytes in the first tanks and the electrolytes in the second tanksto circulate the selected electrolytes through a cell, wherein theelectrolytes reserved in the second tanks are electrolytes having aproportion of a quantity of active material produced in a chargingreaction to a total quantity of active material of not less than 50%.

[0015] During the steady load-leveling operation, the electrolytes inthe first tanks are used to charge and discharge electricity. During theemergency operation such as electric power failure, the electrolytes inthe first tanks are switched to the electrolytes in the second tanks,then discharging electricity. As a result of this, the electrolytes inthe second tanks that are kept high in state of charge are fed to thecell at any time, so that the high overload operation is ensured,regardless of the discharge condition of the first tanks.

[0016] For determining an output value of an electrical overload outputat a high overload rate during the operation, the state of charge of theelectrolytes fed to the cell is a more important factor than a totalcapacity of the electrolytes remaining in the tanks. Due to this, evenwhen a large quantity of electrolyte of a low state of charge iscontained in the tanks, they do not produce the expected output of theoverload.

[0017] In general, a quantity of electrolyte corresponding to thecapacity of the order of eight hours is required for charge or dischargeof electricity for a load-leveling purpose. On the other hand, aquantity of electrolyte corresponding to the capacity of the order oftwo hours at largest is just required for electricity required for anemergency operation such as for example during the time of electricpower failure. Due to this, when the state of charge is tried to bealways kept high by increasing a quantity of electrolyte without theswitching of the electrolyte, as conventionally, a large quantity ofelectrolyte is required and the tanks are also required to be increasedin size. In contrast to this, when the switching of the electrolytesaccording to the present invention is used, a relatively small quantityof electrode is only required for emergency operation and thus thesecond tanks of a smaller size than the first tanks is also required.

[0018] It is to be noted here that the term “a set of” used for both thefirst tanks and the second tanks means that a tank for reserving thepositive electrolyte and a tank for reserving the negative electrode arepaired.

[0019] Electrolyte that is in a substantially fully charged state or ina nearly fully charged state is used for the electrolytes reserved inthe set of second tanks. In other words, the electrolyte of high instate of charge is used therefor. The expression “the electrolyte ishigh in state of charge” indicates the state that when a vanadium-basedelectrolyte is used for the electrolyte, the electrolyte for thepositive electrode has a high ratio “(concentration of quinquevalentvanadium ions)/(concentration of tetravalent+quinquevalent vanadiumions)” and the electrolyte for the negative electrode has a high ratio“(concentration of bivalent vanadium ions)/(concentration ofbivalent+trivalent vanadium ions)”. It is preferable that a ratio of(concentration of quinquevalent vanadium ions)/(concentration oftetravalent+quinquevalent vanadium ions) is the order of not less than50% and a ratio of (concentration of bivalent vanadiumions)/(concentration of bivalent+trivalent vanadium ions) is the orderof not less than 50%.

[0020] Valves are preferably used as the switching means. Preferably,the secondary battery system comprises an association mechanism forcontrolling the switching means on the side on which the electrolytesare discharged from the first tanks or the second tanks and theswitching means on the side on which the electrolytes are fed to thefirst tanks or the second tanks in association with each other. Thisassociated switching operation can allow balancing of a quantity ofelectrolytes discharged from the tanks and a quantity of electrolytesfed to the tanks at the switching of the tanks, to prevent imbalance ofquantity of electrolytes in the cell or generation of considerablepressure change. The associated control of the switching means can beeasily realized by electrically controlling the open/close of thevalves.

[0021] It is preferable that there are provided electrolyte circulationpumps between the switching means on the side on which the electrolytesare discharged from the first tanks or the second tanks and the cell.This arrangement can provide the result that the pumps for the firsttanks and the pumps for the second tanks can be combined for common use.Needless to say, the pumps for feeding the electrolytes from the firsttanks to the cell and the pumps for feeding the electrolytes from thesecond tanks to the cell may be provided separately from each other.

[0022] Further, the present invention provides an operating method of asecondary battery system which in case of emergency operation allows aswitching to electrolytes for emergency operation of at least equal instate of charge to electrolytes for steady operation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a diagrammatic illustration of construction of a redoxflow battery system of the present invention. FIG. 2 is a graph showingthe properties of the redox flow battery when discharging in its fullycharged state. FIG. 3 is a graph showing the properties of the redoxflow battery when discharging from the end stage of discharge. FIG. 4 isa diagrammatic illustration of a part of a shared-pump type of redoxflow battery system of the present invention. FIG. 5 is a diagrammaticillustration of a part of a redox flow battery system having a pluralityof cell stacks of the present invention. FIG. 6 is an explanatory viewof an operating principle of a redox flow battery. FIG. 7 is anillustration of construction of a cell stack of the redox flow battery.

BEST MODE FOR CARRYING OUT THE INVENTION

[0024] In the following, certain preferred embodiments of the presentinvention are described.

First Embodiment

[0025]FIG. 1 is a diagrammatic illustration of construction of a redoxflow battery system of the present invention.

[0026] This battery system comprises a single cell stack 100, two setsof tanks 31, 32 and 33, 34 for reserving electrolytes, valves 41-48 forallowing switching of the electrolytes contained in the tanks 31-34, toselectively supply the electrolytes to the cell stack 100, and pumps51-54 for circulating the electrolytes.

[0027] The cell stack 100 is identical in construction to theconventional one, as illustrated in FIGS. 6 and 7.

[0028] The tanks 31-34 comprise the first tanks 31, 32 for supplying theelectrolytes to the cell stack for the purpose of load-leveling duringthe steady operation and the second tanks 33, 34 for supplying theelectrolytes to the cell stack in an emergency operation, such as duringthe time of electric power failure. The first tanks and the second tankscomprise positive electrolyte tanks 31, 33 and negative electrolytetanks 32, 34, respectively.

[0029] Vanadium-based electrolytes are used for the electrolytesreserved in the first tanks and the second tanks. The positiveelectrolyte contains V⁴⁺/V⁵⁺ ions and the negative electrolyte containsV³⁺/V²⁺ ions.

[0030] Fully charged electrolytes are used for the electrolytes reservedin the set of second tanks 33, 34. The electrolyte having a high ratioof “(concentration of quinquevalent vanadium ions)/(concentration oftetravalent+quinquevalent vanadium ions)” may be used for the positiveelectrode and the electrolyte having a high ratio of “(concentration ofbivalent vanadium ions)/(concentration of bivalent+trivalent vanadiumions)” may be used for the negative electrode.

[0031] There are provided a total of eight valves 41-48, including thevalves 41, 42 for controlling the supply of the electrolyte from the setof first tanks 31, 32 to the cell stack 100, the valves 43, 44 forcontrolling the discharge of the electrolyte from the cell stack 100 tothe set of first tanks 31, 32, the valves 45, 46 for controlling thesupply of the electrolyte from the set of second tanks 33, 34 to thecell stack 100, and the valves 47, 48 for controlling the discharge ofthe electrolyte from the cell stack 100 to the set of second tanks 33,34.

[0032] There are provided a total of four pumps 51-54, including thepump 51 for feeding the positive electrolyte from the first tank 31, thepump 52 for feeding the negative electrolyte from the first tank 32, thepump 53 for feeding the positive electrolyte from the second tank 33,and the pump 54 for feeding the negative electrolyte from the secondtank 34.

[0033] In the redox flow battery system thus constructed, during thesteady operation for load-leveling and the like, the electrolytes in thefirst tanks 31, 32 are used for the charge and discharge of electricity.During this steady operation, the pumps 53, 54 are put in itsinoperative state, with the valves 45-48 closed, while on the otherhand, the pumps 51, 52 are bought into operation, with the valves 41-44opened. In the steady operation, when the electrolytes in the firsttanks are high in state of charge, such as, for example, immediatelyafter electrically charged, the redox flow battery can allow a highoverload operation, while however, at the end stage of discharge orafter completion of discharge, it is too hard for the redox flow batteryto allow the high overload operation.

[0034]FIG. 2 is a graph showing the properties of the redox flow batterywhen discharging in its fully charged state. FIG. 3 is a graph showingthe properties of the redox flow battery when discharging at the endstage of discharge. The graph of FIG. 2 shows a discharge curve plottedwhen a battery having a capability of about two hours at a dischargingrate of 60 mA/cm² is discharged in its fully charged state of 1.55V. Thegraph of FIG. 3 shows a discharge curve plotted when the battery havinga capability of about two hours at a discharging rate of 60 mA/cm² isdischarged for one hour and forty-eight minutes, first, and, then,discharged in its charged state of 1.21V. It will be understood fromcomparison between both graphs that the battery can allow an output at ahigh voltage when it is in the state in which the electrolyte is fullycharged to be high in state of charge, while on the other hand, it canallow substantially no overload operation when it is at an end stage ofdischarge at which the electrolyte is low in state of charge to cause asignificant drop of voltage in the cell leading to a stop of discharge.

[0035] On the other hand, during an emergency operation, such as duringthe time of electric power failure, the electrolytes to be fed to thecell stack 100 are switched from the electrodes in the first tanks 31,32 to the electrodes in the second tanks 33, 34 to dischargeelectricity, so as to allow the high overload operation. The switchingis controlled by closing the valves 41-44 and stopping the pumps 51, 52and, then, opening the valves 45-48 and bringing the pumps 53, 54 intooperation. Since the electrolytes in the second tanks 33, 34 are kepthigh in state of charge, the battery can allow the high overloadoperation at any time, regardless of the state of charge of theelectrolytes in the first tanks.

[0036] It is preferable that when the electrolytes to be fed to the cellstack are switched, switching operation of the valves 41, 42, 45, 46arranged on the side on which the electrolytes are discharged from thefirst and second tanks and switching operation of the valves 43, 44, 47,48 arranged on the side on which the electrolytes are fed to the firstand second tanks are controlled in association with each other. Thisassociated switching operation of the valves can allow balancing of aquantity of electrolytes discharged from the tanks and a quantity ofelectrolytes fed to the tanks at the switching of the tanks, to preventimbalance of quantity of electrolytes in the cell or generation ofconsiderable pressure change.

Second Embodiment

[0037] While in the first embodiment, the pumps 51, 52 and the pumps 53,54 are placed for the electrolytes of the first tanks 31, 32 and thesecond tanks 33, 34, respectively, the pumps for each set of tanks maybe combined for common use. FIG. 4 is a diagrammatic illustration of apart of a shared-pump type of redox flow battery system of the presentinvention. In this illustration, like reference characters refer tocorresponding parts of FIG. 1.

[0038] As illustrated, pumps 55, 56 are interposed between anintermediate part of piping between the valves 41, 45 and the cell stack100 and between an intermediate part of piping between the valves 42, 46and the cell stack 100, respectively, for connection between the valvesand the cell stack. This can allow selective switching between theelectrolytes in the first tanks and the electrolytes in the second tanksand circulation of the selected electrolytes by using a total of twopumps 55, 56.

[0039] The switching operation (open/close operation) of the valves41-48 required for the switching of the electrolytes is identical withthat of the first embodiment.

Third Embodiment

[0040] Further, a diagrammatic illustration of a part of a redox flowbattery system having a plurality of cell stacks 100-102 is shown inFIG. 5. In this third embodiment as well, the selective switchingbetween the electrolytes in the first tanks and the electrolytes in thesecond tanks is controlled by the valves 41-48 being opened and closedin the same manner as in the first embodiment. Thus, the high overloadoperation can be provided, regardless of the state of charge of theelectrolytes in the first tanks.

Capabilities of Exploitation in Industry

[0041] As described above, according to the battery of the presentinvention, there are provided specific tanks for storing theelectrolytes that are constantly kept in the substantially fully chargedstate, in addition to the tanks for electrolytes for used in the steadyload-leveling operation. This can provide the result that in case ofemergency, the required electrolytes can be fed from those specifictanks for the overload operation for any condition for the load-levelingoperation.

1. A secondary battery system comprising: at least one set of firsttanks for reserving electrolytes required for a steady operation, atleast one set of second tanks for reserving electrolytes required for anemergency operation, and switching means for allowing selectiveswitching between the electrolytes in the first tanks and theelectrolytes in the second tanks to circulate the selected electrolytesthrough a cell, wherein the electrolytes reserved in the second tanksfor the steady operation are electrolytes having a proportion of aquantity of active material produced in a charging reaction to a totalquantity of active material of not less than 50%.
 2. The secondarybattery system according to claim 1, which further comprises anassociation mechanism for controlling the switching means on the side onwhich the electrolytes are discharged from the first tanks or the secondtanks and the switching means on the side on which the electrolytes arefed to the first tanks or the second tanks in association with eachother.
 3. The secondary battery system according to claim 1, whereinthere are provided electrolyte circulation pumps between the switchingmeans on the side on which the electrolytes are discharged from thefirst tanks or the second tanks and the cell.
 4. An operating method ofa secondary battery system which in case of emergency operation allows aswitching to electrolytes for an emergency operation of at least equalin state of charge to electrolytes for a steady operation.